When a pupil is detected on an image, error detection may occur frequently because resolution in a pupil part is too low. For example, error detection occurs when a field angle of an imaging apparatus is increased for taking a picture of a wide range and resolution in an eye area cannot be secured sufficiently.
In a typical pupil detection method based on an image, there is utilized a first feature that brightness of a pupil part is lower than brightness of a periphery of the pupil part, or a second feature that a pupil has a circular or ellipsoidal shape. However, when resolution cannot be secured sufficiently, sometimes the contour of the pupil on the image has a polygonal shape and does not have the circular or ellipsoidal shape. When the pupil detection is performed in this situation using the above described second feature, error detection occurs frequently. There are many parts each having a low brightness comparable with that of the pupil part on the image around an eye although the shape thereof is not actually circular or ellipsoidal, such as eyelashes, an iris contour, a shadow caused at an eye tail and an eye inner corner, or noise generated when an S/N ratio is poor. Then, when resolution is low, such a part having a low brightness comparable with that of the pupil part is detected erroneously as the pupil, although the shape thereof is different from the shape of the pupil.
For this problem, there has been proposed the following techniques. For example, in a technique disclosed in Patent Literature 1, an eye area is detected from a face image, and zoom processing is performed on the detected eye area. Then, an image of the eye area enlarged so that the edge of a pupil may be observed sufficiently is obtained and thereby a resolution of the pupil necessary for the pupil detection is secured.
Further, there is a technique not for the pupil detection, but as a technique, which is disclosed in Patent Literature 2, for detecting an iris which is a part of an eye. In the technique disclosed in Patent Literature 2, an iris contour is preliminarily detected from an eye area and the image of the eye area is converted so that the iris contour has a circular shape of a predetermined size.